Transcript Constraining the Kaluza-Klein gluon mass in the Randall

Kaluza-Klein gluon production
at the LHC
Farvah Nazila Mahmoudi
(Uppsala University - Sweden)
in collaboration with M. Guchait and K. Sridhar
From Strings to LHC II – Bangalore – Dec. 20, 2007
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Contents
Randall-Sundrum Model RS1
Modified RS1
Collider search
Kaluza-Klein gluon
Tevatron constraints
LHC constraints
Conclusion
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Brane world scenarios
Idea:
 We are confined on 3-brane
 Graviton lives in the bulk
graviton
Typical scenarios:
 Large (flat) Extra Dimensions
(Arkani-Hamed-Dimopoulos-Dvali,’98)
 Warped (small) Extra Dimensions
(Randall-Sundrum,’99)
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Randall-Sundrum Model (RS1)
Setup:
Five-dimensional theory
Warped extra dimension
A slice of anti-de Sitter spacetime in five dimensions (AdS5)
Fifth dimension  compactified on a S1/Z2 orbifold
The compactification radius Rc is of the order of
Two branes: at orbifold fixed points  = 0 and  = 
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Randall-Sundrum Model (RS1)
Two D3-branes:
 = extra-dimension
bulk
coordinate
Rc = (modest-sized)
radius
UV/Planck brane
IR/TeV brane
5 dimensional metric:
K = mass scale
Farvah Nazila MAHMOUDI
Warp factor
From Strings to LHC II – Bangalore – Dec. 20, 2007
Randall-Sundrum Model (RS1)
The warp factor acts as a conformal factor for the
fields localized on the brane
 Mass factors get rescaled by this factor
Solve the hierarchy!
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Randall-Sundrum Model (RS1)
Brane separation stabilized by a bulk scalar field
Discrete KK-spectrum with masses given by a
common mass factor multiplied by the zeroes of
the Bessel function.
 Interesting collider phenomenology
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Deformation of the original scenario
 AdS/CFT correspondence  RS model is dual to a 4-d
effective theory incorporating gravity
 The dual theory is conformally invariant from the Planck
scale down to the TeV scale
 The KK excitations as well as the fields localized on the TeV
brane are TeV-scale composites
 The original RS theory is dual to a theory of TeV-scale
compositeness of the entire SM
unviable
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Bulk Randall-Sundrum
Modifying the model:
Simplest possibility: modify the model so that only the Higgs
field is localized on the TeV brane while the rest of the SM
fields are in the bulk
 only the Higgs is composite
makes sense because only the scalar sector is natural
guiding principles:
 flavour hierarchy,
 consistency with electroweak precision tests
 avoidance of flavour-changing neutral currents
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Bulk Randall-Sundrum
The location of the fermions in the bulk:
 To get a large Yukawa coupling (overlap with the Higgs) one
needs to localize the fermion close to the TeV brane
 The fermions close to the Planck brane will have small Yukawa
couplings
top sector:
Large Yukawa of the top  proximity to the TeV brane
(t, b)L cannot be close to the TeV brane
tR needs to be localized close to the TeV brane
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Bulk Randall-Sundrum
 To avoid huge effects of FCNCs,
 To be consistent with precision tests of the electroweak
sector:
the masses of the KK modes of the gauge bosons have
to be strongly constrained
bosons are found to be in the region of 2-3TeV
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Production of Kaluza-Klein gluons
The best signal for this scenario is probably:
 production of KK gluons 
gluon KK modes near the TeV brane
Orthonormality of profiles of gluons and KK gluons
 suppressed coupling
 gg at a hadron collider cannot produce the KK gluon at leading
order
 KK gluon can, therefore, be produced by annihilation of light
quarks
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Kaluza-Klein gluon
Non universal couplings to the SM particles
g = gauge coupling
Large BR into right-handed top/anti-top pairs
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Constraint from Tevatron
What is the direct, model-independent bound that
existing collider data can provide ?
KK gluon with a mass just a little above the
branching into top pairs: about 92.5%
threshold: large
To this: add the central value of the SM
production cross-section:
for mt = 175GeV
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Constraint from Tevatron
Tevatron Run II
1.96 TeV in the center of mass
Experimental value:
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Constraint from Tevatron
CTEQ4M
PDFLIB
95 C.L.
Direct bound!
M. Guchait, F.M. and K. Sridhar, JHEP 0705, 103
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
KK gluon at the LHC
Production mechanism:
and
annihilation
Agashe et al., hep-ph/0612015
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
KK gluon at the LHC
For 100 f b-1 :
Discovery of the KK Gluon with M < 4 TeV
Agashe et al., hep-ph/0612015
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Associate production with ttbar
: produced from both gg and
initial states
through the usual QCD processes
KK gluon: radiated from one of the heavy-quark legs
Interesting:
 gg initial state contributes to the associated production
process
 the process directly probes the coupling of the gKK to the
tops which is an important feature of the new dynamics.

The produced gKK decays into a
Farvah Nazila MAHMOUDI
pair
From Strings to LHC II – Bangalore – Dec. 20, 2007
Associate production at the LHC
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Associate production at the LHC
Background:
two non-resonant
pairs coming from QCD processes
 computed using ALPGEN
from gKK : large momenta
other
: more moderate momenta
enhanced (signal / QCD background)
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Associate production at the LHC
Signal kinematics: standard three-jet production
 all three final-state particles in our case are massive
 lower cut of 300 GeV on the pT of the t and the tbar
coming from the decay of the gKK and
 a cut of 50 GeV on the each of the other pair
14 TeV in the center of mass
Assuming 100 fb-1 integrated luminosity
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
KK Gluon at the LHC
Cuts
from gKK : pT > 300 GeV
other
: pT > 50 GeV
significance = 5
 M = 2790 GeV
background cross
section: 0.33 fb
M. Guchait, F.M. and K. Sridhar, arXiv:0710.2234
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
KK Gluon at the LHC
Mass obtained for a
significance of 5 in
function of pT
M. Guchait, F.M. and K. Sridhar, arXiv:0710.2234
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007
Conclusions
Interesting phenomenology from the Bulk Randall-Sundrum model
Striking prediction: existence of KK gluons
with large decay to top/anti-top pair
2 GMS1 results:
GMS1:
Using Tevatron data:
First direct constraints on the KK gluon mass
The lower bound is obtained to be about 770 GeV
GMS2:
Associate production process at the LHC: promising results
LHC will probe masses in the range 2.8-2.9 TeV
next steps: hadron level MC, study of polarizations,…
1
Guchait, Mahmoudi, Sridhar
Farvah Nazila MAHMOUDI
From Strings to LHC II – Bangalore – Dec. 20, 2007